Low temperature polymerization of ethylene



April 1l 1961 P. EHRLlcl-l Erm. 2,979,496A

LOW TEMPERATURE POLYMERIZATION 0F ETHYLENE Filed Feb. 26, 1958 ClTICAL. 'POLYMEEIZATION PPESSUEE OF ETHYLENE PAUL. EHLICH and nIOBET N. PITTILO ATTPNE Y.

United States Patente() LOW kTEMPERATURE POLYMERIZATION OF ETHYLENE `Paul Ehrlich, Hampden, yMass., and Robert N. Pittilo, lTexas City, Tex., assignors to Monsanto Chemical .Companyg St. Louis, Mo., a corporation of Delaware Filed Feb. 26, 1958, Ser. No. 717,701

V3 Claims. (Cl. 26094.9)

`This invention relates'to the high-pressure polymerizaftion of ethylene. vMore particularly, it relates to va l polymerization catalyst rand the polymerization of ethylene therewith at low temperature.

Ethylene polymers formed at low process temperatures, i.e., :100 to 180 C., are known to possess prop- :erties making them .especially desirable for many .-applications. For example, the lower the polymerization-temperature. employed the higher will be the .melting .point,.adensity, crystallinity, rigidity, etc. of the polymers formed. .'Heretofore, ,it has been commercially imprac- ;tical" to :produce vsuch polymers .continuously .by oxygeninitiated yprocesses '(.see Fawcettet al., U.S. 2,153,553)

.isince rthegpolymerization rates at `temperatures below =aboutv180 fC. become inordinatelyslow. Even batch @processes become impractical .-:at .temperatures below about 130 C.

It is an object of this ,invention to provide :aprocess .whereby ethylene may .be'polymerized continuously, nor

:at reasonable batch rates, .at temperatures of fromi100 .-to.180 C. .usingan oxygen initiated system.

IAnotherobject isy toy provide low temperature ethylene `:polymerizates via :an oxygen-initiated process.

Affurther object is'ztov-providea `novel ethylene `poly- ;merization acatalyst -derived in part from rthe .reactionl of foxygen .and ethylene.

These and other objects :are attainedi by rst -prepar- .ing fan `ethylene polymerization ycatalyst Vdissolved in lethylenef'by reacting oxygen `with ethylene inthe proporytionsof from 50.to 10,000 parts by weight of :oxygen vper million parts of -ethyleneat a pressure belowthe :critical polymerizationpressure of said. ethylenevand at a Atemperature of from 130 to 190 C. until .froml30to '-90% of :the oxygen is in chemically combined form.

The catalyst solution `is subsequently intimately vcombined with fresh substantially oxygen-free ethylene Yin such v.proportions .that the linal mixture contains from 10 to 4000 .parts of voxygen per million parts of ethylene.

f=Einally the ethylene therein is polymerized at a pressure above ythe critical polymerization pressure ofsaid ethyleene iat a `temperature of from 100 to `180 C.

'The critical polymerization pressure is Vthat pressure below which ethylene cannot be made to polymerize-in appreciable quantity for given temperatureand total oxygen concentration conditions. 'This phenomenon lis peculiar to oxygen-initiated ethylene Vpolymerizations only. The relationship between the critical polymerization pressure, temperature and total oxygen concentration isshownin the figure. Note that for anygiven 4temperature `the critical polymerization pressure decreases with lincreasing total loxygen concentration;` decreasing sharply at low total oxygen concentrations but eventually Atending toward ra plateau at high total oxygen concentrations.

The following examples are given `in -illustration of '.the inventionand are-not intended as limitations thereof.

Where parts arementionedthey are parts by weight.

2,979,496 i Patented Apr. 11, 1961 ICC EXAMPLE I About 10,000 parts of substantially oxygen-free ethylene and about 5 partsof oxygen are charged in intimate mixture to `a pressure vessel maintained at a pressure 0110,000 p.s.i. The temperature of the reaction mixture is raised to and maintained at ,165 C. for a period of l5 -minutes rafter which time about 50% of the oxygen has entered into chemical combination with a portion of 'the'ethylene Anethylene solution of a catalytic mixture 'of oxygenfand Van `oxygen-ethylene complex is obtained. Similar ethylene polymerization catalyst solutions are obtained using other temperatures, Ypressures `and oxygen Vconcentratiomas shown in Table I:

The ethylene polymerization catalyst solution prepared in Examplel is charged'to a second pressure vessel equipped with external cooling means Where it is intimately ymixed -with rabout 90,000 kparts of substantially oxygen-free ethylene vata pressure of 35,000 p.s.i. and 7a -temperature of 130 C. -Almost instantaneously a sharp drop -in pressure is observed as polymerization occurs; the heat .of polymerization being substantially .completely removed `by cooling `to lavoid an explosive reaction. rAfter 2 .minutes total reaction time the poly- Imerization mixture is Vvcooled to about 50 C. and the pressure is released. A rubbery mass of ethylene poly- ,mer having an average molecular weight, by osmotic ,pressure of .about 170,000 is obtained in about 18% yield. VThe-.polymer meltsat about 119 C., has a density :of about 0.925 gms. per cc. and analyzes Vabout 1.7 .methyl Mgroups per 'carbon atoms in the polymer backbone by infrared spectroscopy.

lExample II is repeated using various temperatures, lpressures :and catalyst concentrations, with results as shown in Table II.

Table Il Cata- Reac- Melt Den- ,.Ex. Press 'Iemp., lyst tion M.W.2 Pt sity,

4p.s.i. C. Conc.l Time (o C' gms./

(Min.) cc.

l Parts of total oxygen per MM puts of ethylene. Number average molecular weight by osmotic pressure by thousands EXAM-PLE III Example II is repeated except that the ethylene polymerization catalyst solution from Example I is cooled to 20 C. and stored at a pressure of 4,000 p.s.i. for Yabout Y24 hours before use. An ethylene polymer is vobtained having substantially identical properties to the polymer obtained-in Example 1I.

For clarity of description, the process of this invention is best divided int'o two basic parts. Part A is concerned vwiththepreparation of the polymerization catalyst solution and Part IB rdeals with'the polymerization of ethylene with the catalyshsolutcn.

envases .million parts of ethylene are employed. The ethylene used-need not be oxygen-free but any oxygen contained therein must be included as partV of the total oxygen employed. Y

As has been stated, the variables of pressure, temperature and oxygen concentration are inter-related and determine the time required to react the desired proportion fof the-oxygen. Generally speaking, the reaction rate varies directly with temperature and with pressure. Ob-

viously, it is most advantageous to prepare this ethylene v polymerization catalyst atthe higher reaction rates. Reaction periods of under 5 minutes are easily obtainable `(see Table I) at temperatures of 165 C. and above, making it commercially practical to produce the ethylene .polymerization catalyst in continuous high pressure reactors. At temperatures below 165 C. batch preparation of the catalyst becomes most practical. Pressures of from 5,000 to 12,000 p.s.i. are generally employed. However, the actual reaction pressure employed mustbe selected consistent with the temperature and catalyst concentration so as not to reach the critical polymerization `pressure of the ethylene.

The exact nature of the ethylene polymerization catalyst of this invention is not known. However, it is known that a portion of the oxygen enters into chemical combination with a portion of the ethylene. It is further known that unless at least 30% of the oxygen is reacted, substantially instantaneous ethylene polymerizations cannot be achieved in the polymerization process. Furthermore, if less than 10% of the total oxygen remains in uncombined form in the catalyst solution when used, the ethylene polymerization rates are again sluggish. Therefore, the first part of the process of this invention produces an ethylene solution of a catalytic mixture of oxygen and oxygen-ethylene reaction products. These catalyst solutions will be stable at the reaction temperatures for-a period of from several minutes to several hours depending upon the reaction conditions, but when cooled to temperatures below 50 C. they become stable for at least 24 hours. The extent ofthe reaction between the oxygen and the ethylene can easily be determined by galvanic cell measurements.

As has .been mentioned, the catalyst preparation procs lends itself equally well to either batch or continuous production depending predominantly upon the temperature employed. In continuous processes, it may be chosen to either pass the catalyst solution into the polymerization reactor as it is formed, or to cool the catalyst solution and pass it into a high pressure storage vessel from which it may be either continuously metered or employed in bulk in subsequent polymerization. In a preferred embodiment it is desired to pass the catalyst solution continuously into the polymerization reactor as described in Part B herein.

PART B The ethylene polymerization catalyst solution prepared as in Part A herein is intimately mixed with fresh' sub- `stantially oxygen-free ethylene in a suitablehigh-pressure vessel in proportions such that from 10 to 4,000 parts of total oxygen, both free and chemically combined, are f present per million parts of ethylene. Both the ethylene presently added and the ethylene `in the catalyst solution are used to determine the total parts of ethylelriel This .mixing step must be effected at a pressure below the critical polymerization pressulrepof the ethylene but subfstantially immediately'thereafter the pressure may heV raised above this critical point if-desired.

vare suitable for use in either continuous polymerization --merization pressure of the ethylene. 'the rate of polymerization varies directly with each of the 20 processes. 'cess employed is essentially governed by the polymer Polymerization is effected in a suitable high pressure vessel equipped with either-internal or external cooling means. Continuous or batch reactors may be employed depending upon the polymers desired and the processing conditions employed. Generally, the polymerizations are conveniently performed under constant pressure conditions, but such is not a limitation since it is onlynecessary that the pressure be maintained at some point intermediate the critical polymerization pressure and the explosive limit of the ethylene polymerization mixture. However, diminishing pressure processes wherein the vpressure decreases as polymerization takes place, may

be employed. Y l

The mixture of ethylene and catalyst is introduced to the reaction vesselrand then brought to the desired polymerization temperature in the range of from to 180 C. and the pressure is increased to above the critical poly- Generally speaking,

variables of temperature, pressure and catalyst concentration. However, at no time should the polymerization conditions and catalyst concentration be permitted to at- -tainthe explosive limits so well-known to those skilled in the art. Therefore, polymerization reactions performed at elevated pressures become hazardous and should be employed only in combination with the lower polymerization temperatures or catalyst concentrations, pressures of about 40,000 p.s.i. being the maximum contemplated.

The heat released due to the exothermic nature of the ethylene polymerization must be at least partially and `preferably totally removed during the polymerization.

This is necessary to the obtainment of the desired lowtemperature polymerizates and also to avoid attaining explosive conditions within the reactor.

Polymerization temperatures of from to 180 C.

processes or batch processes. However, polymerization temperatures between 100 and 120 C. do not produce 'rapid polymerization rates at even the highest pressures and are therefore practically employed only in batch Therefore, the specific polymerization prodesired. However, it is preferred to use continuous poly- -merization whenever possible.

After polymerization to an appreciable yield has been eifected, i.e., from 10 to 25%, the ethylene polymer is i recovered by means conventional to the art. Generally, Yeither of two means may be employed. Releasing the 'pressure at polymerization temperatures generally gives white powdery polymer precipitates. Swollen amorphous polymer masses are recovered by tirst cooling the polymerization mixture and subsequently releasing the pressure.

The physical properties of low-temperature ethylene polymerizates are well-known in the art. For instance,

sity, rigidity, optical clarity, grease resistance, crystallinity,

-etc. Also the degree of short chain branching is directly 60 proportional to the polymerization temperature.

These ethylene polymers may be used in the conven- Ytional polyethylene applications but are most advantage- .ously employed in the applications which capitalize upon their special properties. Such uses include various special- ,ty items including bottles, containers, closures, ,packaging Vfilms, wire and cable coatings, etc.

These polymers also may be modified by the incorporation of conventional additives such as lubricants, stabilizers, anti-oxidants, dyes, pigments, etc. q

It is obvious that many variations may be made in the products and processes set forth above without departing from the spirit and scope of this invention.

polymers which -comprises (1)l preparing an` ethylene.

polymerization catalyst by contacting oxygen with ethylene in a proportion of from to 10,000 parts by weight of oxygen per million parts by weight of ethylene at a pressure below the critical polymerization pressure of said ethylene and at a temperature of from 130 to 190 C. until from 30 to 90% of the oxygen becomes chemically combined, the remaining to 10% of the oxygen remaining uncombined as an active component of said ethylene polymerization catalyst, (2) combining said ethylene polymerization catalyst with further ethylene in such proportion that the total concentration of free plus chemically combined oxygen is from 10 to 4,000 parts by weight per million parts by weight of ethylene, and (3) polymerizing the ethylene at a pressure above the critical polymerization pressure of saidV ethylene at a temperature of from to 180 C.

2. A process as in claim 1 wherein ethylene is continuously polymerized at a temperature of from to C.

3. A process as in claim l wherein ethylene is polymenzed in batch at a temperature of from 100 to 120 C.

References Cited in the iile of this patent UNITED STATES PATENTS Roedel Oct. 22, 1946 Richard et al Sept. 16, 1958 OTHER REFERENCES Organic Chemistry (Astle and Shelton), published by Harper & Brothers, publishers, New York (1952),

15 page 55 relied on. 

1. A PROCESS FOR THE POLYMERIZATION OF ETHYLENE TO SOLID POLYMERS WHICH COMPRISES (1) PREPARING AN ETHYLENE POLYMERIZATION CATALYST BY CONTACTING OXYGEN WITH ETHYLENE IN A PROPORTION OF FROM 50 TO 10,000 PARTS BY WEIGHT OF OXYGEN PER MILLION PARTS BY WEIGHT OF ETHYLENE AT A PRESSURE BELOW THE CRITICAL POLYMERIZATION PRESSURE OF SAID ETHYLENE AND AT A TEMPERATURE OF FROM 130 TO 190*C. UNTIL FROM 30 TO 90% OF THE OXYGEN BECOMES CHEMICALLY COMBINED, THE REMAINING 70 TO 10% OF THE OXYGEN REMAINING UNCOMBINED AS AN ACTIVE COMPONENT OF SAID ETHYLENE POLYMERIZATION CATALYST, (2) COMBINED SAID ETHYLENE POLYMERIZATION CATALYST WITH FURTHER ETHYLENE IN SUCH PROPORTION THAT THE TOTAL CONCENTRATION OF FREE PLUS CHEMICALLY COMBINED OXYGEN IS FROM 10 TO 4,000 PARTS BY WEIGHT PER MILLION PARTS BY WEIGHT OF ETHYLENE, AND (3) POLYMERIZING THE ETHYLENE AT A PRESSURE ABOVE THE CRITICAL POLYMERIZATION PRESSURE OF SAID ETHYLENE AT A TEMPERATURE OF FROM 100 TO 180*C. 